Method for producing barium titanate powder

ABSTRACT

The invention provides a method for producing barium titanate powder comprising the steps of: adding an aqueous slurry of anatase hydrous titanium oxide having a BET specific surface area in the range of 200 m 2 /g to 400 m 2 /g and a half width of diffraction peak of (101) plane in the range of 2.3° to 5.0° as measured by X-ray diffraction to an aqueous solution of barium hydroxide while maintaining the aqueous solution of barium hydroxide at a temperature in the range from 80° C. to the boiling point thereof under normal pressure to cause a reaction of the barium hydroxide with the hydrous titanium oxide to provide an aqueous slurry of barium titanate precursor; and subjecting the barium titanate precursor thus obtained to hydrothermal treatment over a period of not less than 24 hours to provide barium titanate particles.

TECHNICAL FIELD

The present invention relates to a method for producing barium titanatepowder, more specifically to a method for producing barium titanatepowder which is comprised of uniform fine particles, high intetragonality, and superior in dispersibility.

BACKGROUND ART

In recent years, a substantial improvement of the properties of elementsconfiguring the electronic devices and the starting materials formanufacturing the electronic devices has been strongly demanded with areduction in size, an enhancement of performance, and a reduction inweight of various electronic devices.

For example, a multilayer ceramic capacitor (MLCC) has been increasinglystrongly demanded to reduce the layer thickness. Therefore, bariumtitanate for use in dielectric layers in the MLCC has been increasinglydemanded to be uniform fine particles and to be high in tetragonalityand to be superior in dispersibility, for example.

Heretofore, as a method for producing barium titanate, a solid phasemethod, an oxalic acid method, a sol-gel method, and the like are known.However, in order to produce barium titanate fine particles,particularly those having a particle diameter of about 250 nm or less,preferably about 150 nm or less, for satisfying the recent demand forthe reduction in layer thickness of the MLCC, a hydrothermal processwhich is a wet process is advantageous. Since the solid phase method andthe oxalic acid method include a calcination process, uniform particlesare difficult to obtain, and moreover the resulting particles are easyto aggregate so that fine particles are difficult to obtain. The sol-gelmethod employs an expensive alkoxide as the raw material, and thereforethe sol-gel method has a problem in the production cost.

Various methods for producing barium titanate by the hydrothermalprocess are heretofore already known. By way of example, a method ismentioned in which an aqueous solution of barium salt is added to andreacted with a slurry of hydrous titanium oxide in the presence of acarboxylic acid to produce barium titanate core particles, the slurrycontaining the barium titanate core particles thus obtained is subjectedto hydrothermal treatment to obtain spherical barium titanate particles,and then the obtained spherical barium titanate particles are calcinedat a temperature of 800° C. to 1200° C. (Patent Literature 1).

According to this method, barium titanate fine particles having arelatively large BET specific surface area and a high tetragonality canbe obtained. However, as the method requires calcination of bariumtitanate core particles, it is difficult to obtain particles having auniform particle diameter. It is also difficult to obtain particles offine particles because of aggregation of particles. The use of suchbarium titanate as a dielectric material in the manufacturing of MLCCposes difficulty in satisfying the demand of reducing the layerthickness of MLCC.

CITATION LIST Patent Literature Patent Literature 1: JP 2002-211926ASUMMARY OF INVENTION Technical Problem

The invention has been completed in order to solve the above-mentionedproblems involved in the production of barium titanate powder. It is anobject of the invention to provide a method for producing bariumtitanate powder which is comprised of uniform fine particles high intetragonality and superior in dispersibility.

Solution to Problem

The invention provides a method for producing barium titanate powdercomprising the steps of:

adding an aqueous slurry of anatase hydrous titanium oxide having a BETspecific surface area in the range of 200 m²/g to 400 m²/g and a halfwidth of diffraction peak of (101) plane in the range of 2.3° to 5.0° asmeasured by X-ray diffraction to an aqueous solution of barium hydroxidewhile maintaining the aqueous solution of barium hydroxide at atemperature in the range from 80° C. to the boiling point thereof undernormal pressure to cause a reaction of the barium hydroxide with thehydrous titanium oxide to provide an aqueous slurry of barium titanateprecursor; and

subjecting the barium titanate precursor thus obtained to hydrothermaltreatment over a period of not less than 24 hours to provide bariumtitanate particles.

A particularly preferable method for producing barium titanate powderaccording to the invention comprises the steps of:

adding an aqueous solution of titanium tetrachloride and an aqueoussolution of an alkali at the same time to water maintained at atemperature of 45° C. to 65° C. while maintaining the temperature of theresulting reaction mixture in the range of 45° C. to 65° C. andmaintaining the pH thereof in the range of 1.5 to 3.5 to simultaneouslyneutralize the titanium tetrachloride with the alkali thereby to providean aqueous slurry of anatase hydrous titanium oxide having a BETspecific surface area in the range of 200 m²/g to 400 m²/g and a halfwidth of diffraction peak of (101) plane in the range of 2.3° to 5.0° asmeasured by X-ray diffraction;

adding the aqueous slurry of anatase hydrous titanium oxide thusobtained to an aqueous solution of barium hydroxide while maintainingthe aqueous solution of barium hydroxide at a temperature in the rangefrom 80° C. to the boiling point thereof under normal pressure to causea reaction of the barium hydroxide with the hydrous titanium oxide toprovide an aqueous slurry of barium titanate precursor; and

subjecting the barium titanate precursor thus obtained to hydrothermaltreatment over a period of not less than 24 hours to provide bariumtitanate particles.

According to the invention, it is preferred that in the step ofhydrothermal treatment mentioned above, that is, in the step ofhydrothermally treating the barium titanate precursor to provide bariumtitanate particles, the hydrothermal treatment temperature is 250° C. orless.

Advantageous Effects of Invention

According to the method of the invention, there is obtained bariumtitanate powder which is comprised of uniform fine particles high intetragonality and superior in dispersibility.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a scanning electron photomicrograph (50000 magnifications) ofan example of barium titanate obtained by the method of the invention.

DESCRIPTION OF EMBODIMENTS

The method for producing barium titanate powder of the inventioncomprises the steps of:

adding an aqueous slurry of anatase hydrous titanium oxide having a BETspecific surface area in the range of 200 m²/g to 400 m²/g and a halfwidth of diffraction peak of (101) plane in the range of 2.3° to 5.0° asmeasured by X-ray diffraction to an aqueous solution of barium hydroxidewhile maintaining the aqueous solution of barium hydroxide at atemperature in the range from 80° C. to the boiling point thereof undernormal pressure to cause a reaction of the barium hydroxide with thehydrous titanium oxide to provide an aqueous slurry of barium titanateprecursor; and

subjecting the barium titanate precursor thus obtained to hydrothermaltreatment over a period of not less than 24 hours to provide bariumtitanate particles.

When the BET specific surface area of the anatase hydrous titanium oxideused is larger than 400 m²/g, the resulting barium titanate is inferiorin tetragonality. When the BET specific surface area of the anatasehydrous titanium oxide used is smaller than 200 m²/g, the anatasehydrous titanium oxide has a high crystallinity, but on the other hand,it has a poor reactivity with barium hydroxide with the consequence thatthe barium titanate particles obtained are inferior in tetragonality.

In particular, according to the invention, the anatase hydrous titaniumoxide used has a BET specific surface area preferably in the range of200 m²/g to 350 m²/g, and more preferably in the range of 220 m²/g to330 m²/g.

Further, when the anatase hydrous titanium oxide used has a half widthof diffraction peak of (101) plane smaller than 2.3° as measured byX-ray diffraction, it has a high crystallinity, but has a poorreactivity with barium hydroxide, and therefore the resulting bariumtitanate particles are low in tetragonality. On the other hand, when theanatase hydrous titanium oxide used has a half width larger than 5.0°,the resulting barium titanate particles are low in tetragonality.

In particular, the anatase hydrous titanium oxide used in the inventionhas a half width of diffraction peak of (101) plane preferably in therange of 2.3° to 4.0° as measured by X-ray diffraction, and morepreferably in the range of 2.3° to 3.5°.

As described above, the aqueous slurry of anatase hydrous titanium oxidehaving a BET specific surface area in the range of 200 m²/g to 400 m²/gand a half width of diffraction peak of (101) plane in the range of 2.3°to 5.0° as measured by X-ray diffraction is obtained preferably byadding an aqueous solution of titanium tetrachloride and an aqueoussolution of an alkali each maintained at a temperature in the range of45° C. to 65° C. at the same time to water maintained at a temperatureof 45° C. to 65° C. beforehand to simultaneously neutralize the titaniumtetrachloride with the alkali while maintaining the pH of the resultingreaction mixture in the range of 1.5 to 3.5, preferably in the range of2.0 to 3.5.

Therefore, according to the invention, the most preferred method forproducing barium titanate powder comprises the steps of:

adding an aqueous solution of titanium tetrachloride and an aqueoussolution of an alkali at the same time to water maintained at atemperature of 45° C. to 65° C. while maintaining the temperature of theresulting reaction mixture in the range of 45° C. to 65° C. andmaintaining the pH thereof in the range of 1.5 to 3.5 to simultaneouslyneutralize the titanium tetrachloride with the alkali thereby to providean aqueous slurry of anatase hydrous titanium oxide having a BETspecific surface area in the range of 200 m²/g to 400 m²/g and a halfwidth of diffraction peak of (101) plane in the range of 2.3° to 5.0° asmeasured by X-ray diffraction;

adding the aqueous slurry of anatase hydrous titanium oxide thusobtained to an aqueous solution of barium hydroxide while maintainingthe aqueous solution of barium hydroxide at a temperature in the rangefrom 80° C. to the boiling point thereof under normal pressure to causea reaction of the barium hydroxide with the hydrous titanium oxide toprovide an aqueous slurry of barium titanate precursor; and

subjecting the barium titanate precursor thus obtained to hydrothermaltreatment over a period of not less than 24 hours to provide bariumtitanate particles.

According to the invention, as described above, the aqueous slurry ofanatase hydrous titanium oxide is obtained preferably by simultaneouslyneutralizing titanium tetrachloride with an alkali. Herein, thesimultaneous neutralization of the titanium tetrachloride with thealkali refers to simultaneously adding an aqueous solution of titaniumtetrachloride and an aqueous solution of alkali into a containercontaining water and mixing them in the container to neutralize thetitanium tetrachloride with the alkali.

In the simultaneous neutralization of the titanium tetrachloride withthe alkali described above, the concentration of the hydrous titaniumoxide in the slurry when the simultaneous neutralization has beencompleted is not particularly limited, and it is usually preferred thatit is in the range of 10 g/L to 50 g/L in terms of TiO₂. In case thatthe concentration of the hydrous titanium oxide in the slurry is higherthan 50 g/L in terms of TiO₂ when the simultaneous neutralization hasbeen completed, amorphous hydrous titanium oxide is likely to generate,and therefore it becomes difficult to obtain anatase hydrous titaniumoxide. On the other hand, in case that the concentration of the hydroustitanium oxide in the slurry is lower than 10 g/L in terms of TiO₂ whenthe simultaneous neutralization has been completed, the productivity islikely to be poor.

Further, in the simultaneous neutralization of the titaniumtetrachloride with the alkali described above, the temperatures ofwater, the aqueous solution of titanium tetrachloride, and the aqueoussolution of the alkali do not need to be all the same, but thetemperatures are preferably close to each other, most preferably all thesame, in particular.

As the aqueous solution of the alkali solution, an aqueous solution ofan alkali metal hydroxide such as sodium hydroxide and potassiumhydroxide, ammonia, and the like are preferably used. In place of theaqueous solution of alkali, a solid alkali compound may be directlyadded.

Even if the pH of the resulting reaction mixture is in the range of 1.5to 3.5 in the simultaneous neutralization of the titanium tetrachloridewith the alkali described above, when the neutralization temperature islower than 45° C., the anatase hydrous titanium oxide cannot beobtained, and the BET specific surface area of the resulting hydroustitanium oxide exceeds 400 m²/g.

When the neutralization temperature is higher than 65° C., the titaniumtetrachloride is hydrolyzed, and rutile hydrous titanium oxide is likelyto generate or the half width of the hydrous titanium oxide becomessmaller than 2.3°. The rutile hydrous titanium oxide has a poorreactivity with barium hydroxide and the barium titanate particlesobtained are inferior in tetragonality. Also when the half width of thehydrous titanium oxide is smaller than 2.3°, the barium titanateparticles obtained are inferior in tetragonality.

On the other hand, even if the neutralization temperature is in therange of 45° C. to 65° C., when the pH of the resulting reaction mixtureis higher than 3.5, the anatase hydrous titanium oxide cannot beobtained and the BET specific surface area of the resulting hydroustitanium oxide exceeds 400 m²/g. When using such hydrous titanium oxide,the barium titanate particles obtained are inferior in tetragonality.

When the pH of the resulting reaction mixture is lower than 1.5, a largeamount of chloride ions remain as impurities in the hydrous titaniumoxide produced, and as a result, the reactivity thereof with bariumhydroxide deteriorates and thus the barium titanate particles obtainedare inferior in tetragonality.

Thus, according to the invention, titanium tetrachloride issimultaneously neutralized with the alkali in water to provide hydroustitanium oxide, the resulting slurry is filtered and washed with waterto remove therefrom the chloride ions and the like generated in theneutralization, and the resulting cake is dispersed in water, therebythe aqueous slurry of hydrous titanium oxide is obtained which can bepreferably used in the step of obtaining an aqueous slurry of the bariumtitanate precursor.

The aqueous slurry of anatase hydrous titanium oxide having a BETspecific surface area in the range of 200 m²/g to 400 m²/g and a halfwidth of diffraction peak of (101) plane in the range of 2.3° to 5.0° asmeasured by X-ray diffraction is added to an aqueous solution of bariumhydroxide while maintaining the aqueous solution of barium hydroxide ata temperature in the range from 80° C. to the boiling point thereofunder normal pressure to cause a reaction of the barium hydroxide withthe hydrous titanium oxide as described above to provide an aqueousslurry of barium titanate precursor.

The barium titanate precursor obtained by the reaction of the bariumhydroxide with the hydrous titanium oxide is usually comprised of fineparticles of barium titanate having a BET specific surface area in therange of 50 m²/g to 200 m²/g. Since the reaction between the bariumhydroxide and the hydrous titanium oxide is a reaction under normalpressure, the barium titanate precursor obtained has a Ba/Ti ratiosmaller than 1, but it has been confirmed by powder X-ray diffractionthat the barium titanate precursor obtained has a crystal structure ofbarium titanate.

In the aqueous solution of barium hydroxide, the solvent may contain awater-soluble organic solvent insofar as the reaction of the bariumhydroxide with the hydrous titanium oxide is not adversely affected. Assuch a water-soluble organic solvent, ethylene glycol, diethyleneglycol, polyethylene glycol, and the like can be mentioned, for example.

When obtaining the barium titanate precursor by the reaction of thebarium hydroxide with the hydrous titanium oxide, it is preferred thatthe barium hydroxide and the hydrous titanium oxide are used in such amanner that the Ba/Ti molar ratio is in the range of 1.1 to 3.0 when theaddition of the aqueous slurry of anatase hydrous titanium oxide to theaqueous barium hydroxide solution has been completed. In the reaction ofthe barium hydroxide with the hydrous titanium oxide, when the Ba/Timolar ratio is smaller than 1.1, the alkalinity of the resultingreaction system is low so that the reactivity between the bariumhydroxide and the hydrous titanium oxide deteriorates. When the Ba/Timolar ratio is higher than 3.0, the reactivity between the bariumhydroxide and the hydrous titanium oxide has no problems; however, sincethe barium hydroxide which does not contribute to the reaction isexcessively used, there is caused a problem that the production costbecomes high.

In the step of obtaining the aqueous slurry of the barium titanateprecursor described above, the temperature at which the barium hydroxideis reacted with the hydrous titanium oxide is important. Even if theanatase hydrous titanium oxide has a BET specific surface area in therange of 200 m²/g to 400 m²/g and a half width in the range of 2.3° to5.0° as described above, when the temperature of the reaction thereofwith the barium hydroxide is lower than 80° C. under normal pressure,barium titanate particles with a high tetragonality cannot be obtained.The upper limit of the reaction temperature is a temperature up to theboiling point of the reaction mixture containing the barium hydroxide.

According to the method of the invention, the barium titanate precursorobtained as described above is subjected to hydrothermal treatment overa period of not less than 24 hours, the resulting slurry is filtered,washed with water, and then dried, to provide the intended bariumtitanate powder comprised of uniform fine particles superior intetragonality. If it is intended to synthesize barium titanate only bythe hydrothermal treatment without passing through the barium titanateprecursor, the reaction of the barium hydroxide with hydrous titaniumoxide progresses simultaneously with the particle growth of theresulting barium titanate particles, and, as a result, a large amount ofhydroxyl groups are taken into the barium titanate particles, whichmakes it difficult to obtain barium titanate particles with a hightetragonality.

The aqueous slurry of the barium titanate precursor subjected tohydrothermal treatment preferably contains the barium titanate precursorin a concentration in the range of 0.4 mol/L to 1.0 mol/L in terms ofBaTiO₃.

The aqueous slurry of the barium titanate precursor may be subjected tothe hydrothermal treatment while maintaining the obtained concentrationwithout condensing or diluting the same.

The aqueous slurry of the barium titanate precursor is put into anautoclave usually as it is, and is then subjected to the hydrothermaltreatment at a temperature that exceeds the boiling point thereof undernormal pressure and that is usually not more than 250° C., preferably inthe range of 105° C. to 250° C.

In the method of the invention, the hydrothermal treatment is performedover a period of 24 hours or more, usually in the range of 24-1000hours, preferably in the range of 24-500 hours, and most preferably inthe range of 24-200 hours. According to the invention, the particlediameter of barium titanate obtained can be controlled by thetemperature at which and time for which the hydrothermal treatment isperformed.

That is, the particle diameter of the barium titanate increases withincreasing temperature at which the hydrothermal treatment is performed,while the particle diameter of the barium titanate increases withincreasing time for which the hydrothermal treatment is performed. Inparticular, according to the present invention, there is obtained bariumtitanate larger in particle diameter and higher in tetragonality byperforming the hydrothermal treatment for a longer time of period. Whenthe hydrothermal treatment is performed for a period of shorter than 24hours, the resulting particles of barium titanate do not growsufficiently to be low in tetragonality.

After the aqueous slurry of the barium titanate precursor has beenhydrothermally treated in this way, the content in the autoclave iscooled to normal temperature, the obtained slurry is filtered, washedwith water, and then dried to obtain barium titanate powder. The dryingtemperature is usually in the range of 100° C. to 150° C.

The barium titanate powder thus obtained is comprised of uniform fineparticles superior in tetragonality, and it usually has a BET specificsurface area in the range of 3 m²/g to 70 m²/g, preferably in the rangeof 4 m²/g to 20 m²/g.

In the invention, the tetragonality of the barium titanate is evaluatedon the basis of c/a ratio, which can be determined from powder X-raydiffraction of the barium titanate powder. The barium titanate powderobtained according to the invention has a c/a ratios of 1.008 or more,the tetragonality in that sense. Thus, the barium titanate powderobtained is superior in tetragonality.

As set forth above, because the barium titanate obtained in this way hasa high tetragonality only by the hydrothermal treatment of bariumtitanate precursor, there is no need for the barium titanate obtained tobe further calcined and disintegrated.

As described above, the particles of barium titanate obtained by theinvention are high in tetragonality and superior in dispersibility.Further, the particles of barium titanate high in tetragonality are alsohigh in dielectricity, and therefore is suitably used as a material fordielectrics. As a consequence, the barium titanate powder obtained bythe invention can be suitably used as a dielectric material to meet thedemand of reducing the layers thickness in the manufacturing of theMLCC.

The invention is described in detail with reference to Examples, but thepresent invention is not limited to Examples.

EXAMPLES Example 1 Preparation of Aqueous Slurry of Hydrous TitaniumOxide

500 mL of pure water was put into a beaker and then warmed to 55° C. 350mL of an aqueous solution of titanium tetrachloride (manufactured byOsaka Titanium Technologies Co., Ltd., 3.8 mol/L in terms of TiO₂) and 7L of pure water were added at the same time to the water at a rate of2.5 mL/minute and 50 mL/minute, respectively, while simultaneously anaqueous sodium hydroxide solution having a concentration of 30% byweight was also added to the water, and while maintaining the resultingmixture at a pH of 2.5 and at a temperature of 55° C., to simultaneouslyneutralize the titanium tetrachloride with the alkali, the sodiumhydroxide, thereby an aqueous slurry of hydrous titanium oxide having aconcentration of 14 g/L in terms of TiO₂ was obtained.

The aqueous slurry thus obtained was filtered and then washed with waterto remove sodium ions and chloride ions therefrom. Pure water was addedto the obtained cake to provide an aqueous slurry of anatase hydroustitanium oxide having a concentration of 110 g/L in terms of TiO₂.

(Preparation of Aqueous Slurry of Barium Titanate Precursor)

567 mL of pure water and 959 g of barium hydroxide octahydrate(manufactured by Sakai Chemical Industry Co., Ltd.) were put into a 5L-capacity reaction vessel, and then heated to 100° C. to dissolve thebarium hydroxide octahydrate in water to prepare an aqueous bariumhydroxide solution.

The aqueous slurry of the hydrous titanium oxide was added whilemaintaining the temperature at 100° C. to the aqueous solution of bariumhydroxide while maintaining the temperature at 100° C. in 1 hour, andthey were reacted at a temperature of 100° C. for 2 hours, thereby anaqueous slurry of barium titanate precursor having a concentration of0.66 mol/L in terms of BaTiO₃ was obtained. The Ba/Ti molar ratio in theresulting reaction mixture was 2.3 when the addition of the aqueousslurry of hydrous titanium oxide to the aqueous barium hydroxidesolution was completed.

(Hydrothermal Treatment of Aqueous Slurry of Barium Titanate Precursor)

The aqueous slurry of the barium titanate precursor having aconcentration of 0.66 mol/L in terms of BaTiO₃ obtained above was placedin an autoclave vessel, and then subjected to hydrothermal treatment at180° C. for 120 hours. Then, the content in the autoclave was allowed tocool to room temperature. The obtained aqueous slurry was filtered,washed with water, and then dried at 130° C. to give barium titanatepowder.

FIG. 1 is a scanning electron microphotograph (50000 magnifications) ofbarium titanate powder obtained above. The invention provides bariumtitanate powder comprised of uniform fine particles.

Example 2

Barium titanate powder was obtained in the same manner as in Example 1except that the aqueous slurry of the barium titanate precursor washydrothermally treated at a temperature of 180° C. for 48 hours.

Example 3

Barium titanate powder was obtained in the same manner as in Example 1except that the aqueous slurry of the barium titanate precursor washydrothermally treated at a temperature of 200° C. for 24 hours.

Example 4

Barium titanate powder was obtained in the same manner as in Example 1except that the aqueous slurry of the barium titanate precursor washydrothermally treated at a temperature of 160° C. for 168 hours.

Example 5

Barium titanate powder was obtained in the same manner as in Example 1except that the aqueous slurry of the barium titanate precursor washydrothermally treated at a temperature of 200° C. for 180 hours.

Example 6

Barium titanate powder was obtained in the same manner as in Example 1except that the simultaneous neutralization of titanium tetrachloridewas performed at a pH of 3.5 and at a temperature of 45° C., and thehydrothermal treatment of barium titanate precursor was performed at atemperature of 170° C. for 96 hours.

Example 7

Barium titanate powder was obtained in the same manner as in Example 1except that the simultaneous neutralization of titanium tetrachloridewas performed at a pH of 2.0 and at a temperature of 45° C., and thehydrothermal treatment of barium titanate precursor was performed at atemperature of 200° C. for 24 hours.

Comparative Example 1 Preparation of Aqueous Slurry of Hydrous TitaniumOxide

500 mL of pure water was put into a beaker, and was maintained at atemperature of 60° C. 350 mL of an aqueous solution of titaniumtetrachloride (manufactured by Osaka Titanium Technologies Co., Ltd.,3.8 mol/L in terms of TiO₂) and 7 L of pure water were added at the sametime to the water at a rate of 2.5 mL per minute and 50 mL per minute,respectively, while simultaneously an aqueous solution of sodiumhydroxide having a concentration of 30% by weight was added to the waterand while simultaneously the resulting mixture was adjusted at a pH of2.0 and at a temperature of 60° C., to simultaneously neutralize thetitanium tetrachloride, thereby an aqueous slurry of hydrous titaniumoxide having a concentration of 14 g/L in terms of TiO₂ was obtained.

The aqueous slurry was filtered and then washed with water to removesodium ions and chloride ions therefrom. Pure water was added to theobtained cake to provide an aqueous slurry of anatase hydrous titaniumoxide having a concentration of 110 g/L in terms of TiO₂.

(Preparation of Aqueous Slurry of Barium Titanate Precursor)

567 mL of pure water and 959 g of barium hydroxide octahydrate(manufactured by Sakai Chemical Industry Co., Ltd.) were put into a 5L-capacity reaction vessel, and then heated to 100° C. to dissolve thebarium hydroxide octahydrate in water to prepare an aqueous bariumhydroxide solution.

The aqueous slurry of the hydrous titanium oxide was added whilemaintaining the temperature at 100° C. to the aqueous solution of bariumhydroxide while maintaining the temperature at 100° C. in 1 hour, andthey were reacted at a temperature of 100° C. for 2 hours, thereby anaqueous slurry of barium titanate precursor having a concentration of0.66 mol/L in terms of BaTiO₃ was obtained. The Ba/Ti molar ratio in theresulting reaction mixture was 2.3 when the addition of the aqueousslurry of hydrous titanium oxide to the aqueous barium hydroxidesolution was completed.

(Hydrothermal Treatment of Aqueous Slurry of Barium Titanate Precursor)

The aqueous slurry of the barium titanate precursor having aconcentration of 0.66 mol/L in terms of BaTiO₃ obtained above was placedin an autoclave vessel, and then subjected to hydrothermal treatment at190° C. for 0.5 hours. Then, the content in the autoclave was allowed tocool to room temperature. The obtained aqueous slurry was filtered,washed with water, and then dried at 130° C. to give barium titanatepowder.

Comparative Example 2

Barium titanate powder was obtained in the same manner as in ComparativeExample 1 except that the simultaneous neutralization of titaniumtetrachloride was performed at a temperature of 50° C., and thehydrothermal treatment of barium titanate precursor was performed at atemperature of 180° C. for 20 hours.

Comparative Example 3

Barium titanate powder was obtained in the same manner as in ComparativeExample 1 except that the simultaneous neutralization of titaniumtetrachloride was performed at a temperature of 50° C., and thehydrothermal treatment of barium titanate precursor was performed at atemperature of 205° C. for 2 hours.

Comparative Example 4

Barium titanate powder was obtained in the same manner as in ComparativeExample 1 except that the simultaneous neutralization of titaniumtetrachloride was performed at a pH of 3.0, reaction of hydrous titaniumoxide and barium hydroxide was performed at a temperature of 100° C. for5 hours, and the hydrothermal treatment of barium titanate precursor wasperformed at a temperature of 130° C. for 0.5 hours.

Comparative Example 5

Barium titanate powder was obtained in the same manner as in ComparativeExample 1 except that the simultaneous neutralization of titaniumtetrachloride was performed at a temperature of 50° C., the reaction ofhydrous titanium oxide and barium hydroxide was performed at atemperature of 80° C. for 5 hours, and the hydrothermal treatment ofbarium titanate precursor was performed at a temperature of 200° C. for2 hours.

Comparative Example 6

Barium titanate powder was obtained in the same manner as in ComparativeExample 1 except that the simultaneous neutralization of titaniumtetrachloride was performed at a pH of 3.0, the reaction of hydroustitanium oxide and barium hydroxide was performed at a temperature of95° C. for 5 hours, and the hydrothermal treatment of barium titanateprecursor was performed at a temperature of 180° C. for 0.5 hours.

Comparative Example 7

Barium titanate powder was obtained in the same manner as in ComparativeExample 1 except that the simultaneous neutralization of titaniumtetrachloride was performed at a temperature of 50° C., the reaction ofhydrous titanium oxide and barium hydroxide was performed at atemperature of 100° C. for 2 hours, and the hydrothermal treatment ofbarium titanate precursor was performed at a temperature of 110° C. for2 hours.

Comparative Example 8

Barium titanate powder was obtained in the same manner as in ComparativeExample 1 except that the simultaneous neutralization of titaniumtetrachloride was performed at a pH of 5.0 and at a temperature of 20°C., the reaction of hydrous titanium oxide and barium hydroxide wasperformed at a temperature of 70° C. for 2 hours, and the hydrothermaltreatment of barium titanate precursor was performed at a temperature of150° C. for 160 hours.

Comparative Example 9

Barium titanate powder was obtained in the same manner as in ComparativeExample 1 except that the simultaneous neutralization of titaniumtetrachloride was performed at a pH of 2.5 and at a temperature of 55°C., and the hydrothermal treatment of barium titanate precursor wasperformed at a temperature of 250° C. for 2 hours.

Comparative Example 10

Barium titanate powder was obtained in the same manner as in ComparativeExample 1 except that the simultaneous neutralization of titaniumtetrachloride was performed at a temperature of 45° C., and thehydrothermal treatment of barium titanate precursor was performed at atemperature of 180° C. for 20 hours.

Comparative Example 11

Barium titanate powder was obtained in the same manner as in ComparativeExample 1 except that the simultaneous neutralization of titaniumtetrachloride was performed at a temperature of 80° C., and thehydrothermal treatment of barium titanate precursor was performed at atemperature of 200° C. for 24 hours.

Comparative Example 12

Barium titanate powder was obtained in the same manner as in ComparativeExample 1 except that the simultaneous neutralization of titaniumtetrachloride was performed at a pH of 1.5 and at a temperature of 20°C., and the hydrothermal treatment of barium titanate precursor wasperformed at a temperature of 160° C. for 40 hours.

Comparative Example 13

Barium titanate powder was obtained in the same manner as in ComparativeExample 1 except that the simultaneous neutralization of titaniumtetrachloride was performed at a pH of 4.0 and at a temperature of 40°C., and the hydrothermal treatment of barium titanate precursor wasperformed at a temperature of 170° C. for 24 hours.

The reaction conditions in the above-described Examples 1-7 andComparative Examples 1-13, i.e., the pH and temperature at whichtitanium tetrachloride was simultaneously neutralized with sodiumhydroxide, the reaction temperature at which and the reaction time forwhich the hydrous titanium oxide was reacted with barium hydroxide toprovide the barium titanate precursor, and the temperature at which andthe time for which the aqueous slurry of barium titanate precursor washydrothermally treated are shown in Table 1.

The properties of the hydrous titanium oxide used in Examples 1-7 andComparative Examples 1-13, and the properties of the barium titanatepowder obtained in Examples 1-7 and Comparative Examples 1-13 are shownin Table 2.

(Crystal Structure of Hydrous Titanium Oxide)

A small amount of the aqueous slurry of hydrous titanium oxide obtainedby simultaneously neutralizing titanium tetrachloride with sodiumhydroxide was withdrawn, filtered and washed with water, and dried toprovide powder. The crystal structure of the powder was examined using apowder X-ray diffraction apparatus (RINT-TTR III, manufactured by RigakuCorporation, Radiation source: CuKα).

Further, the half width was calculated from the peak of (101) plane andwas used as the index of the crystallinity of the hydrous titaniumoxide. As shown in Table 2, the peak of the hydrous titanium oxide ineach of Comparative Examples 8, 12 and 13 was found to be very broad,and thus the hydrous titanium oxides obtained therein were judged to beamorphous.

(c/a Ratio of Barium Titanate)

The barium titanate powder was subjected to powder X-ray diffractionusing a powder X-ray diffraction apparatus (RINT-TTR III, manufacturedby Rigaku Corporation, Radiation source: CuKα), and then the c/a ratiowas determined using the WPPF method.

(Specific Surface Areas of Hydrous Titanium Oxide Powder and BariumTitanate Powder)

The specific surface area of each of the hydrous titanium oxide powderand the barium titanate powder was measured by a single point BET methodusing a fully automatic specific surface area meter (HM Model-1220,manufactured by Mountech Co., Ltd.), after degassing at 205° C. for 30minutes.

TABLE 1 Preparation of Preparation of Hydrothermal Hydrous BariumTitanate Treatment Titanium Oxide Precursor Temperature Temper- Temper-Temper- ature ature Time ature Time pH (° C.) (° C.) (h) (° C.) (h)Example 1 2.5 55 100 2.0 180 120 Example 2 2.5 55 100 2.0 180 48 Example3 2.5 55 100 2.0 200 24 Example 4 2.5 55 100 2.0 160 168 Example 5 2.555 100 2.0 200 180 Example 6 3.5 45 100 2.0 170 96 Example 7 2.0 45 1002.0 200 24 Comparative 1 2.0 60 100 2.0 190 0.5 Comparative 2 2.0 50 1002.0 180 20 Comparative 3 2.0 50 100 2.0 205 2.0 Comparative 4 3.0 60 1005.0 130 0.5 Comparative 5 2.0 50 80 5.0 200 2.0 Comparative 6 3.0 60 955.0 180 0.5 Comparative 7 2.0 50 100 2.0 110 2.0 Comparative 8 5.0 20 702.0 150 160 Comparative 9 2.5 55 100 2.0 250 2.0 Comparative 10 2.0 45100 2.0 180 20 Comparative 11 2.0 80 100 2.0 200 24 Comparative 12 1.520 100 2.0 160 40 Comparative 13 4.0 40 100 2.0 170 24

TABLE 2 Hydrous Titanium Oxide Barium Titanate Specific Specific SurfaceHalf Surface Area Crystal Width Area (m²/g) Structure*⁾ (Degrees) (m²/g)c/a Example 1 255 A 2.5 7.4 1.010 Example 2 255 A 2.5 10.5 1.008 Example3 249 A 2.4 11.7 1.008 Example 4 249 A 2.4 10.1 1.009 Example 5 255 A2.5 4.9 1.010 Example 6 322 A 3.1 8.7 1.008 Example 7 284 A 3.0 8.61.008 Comparative 1 270 A 2.5 20.6 1.006 Comparative 2 256 A 2.5 11.81.007 Comparative 3 256 A 2.5 17.1 1.006 Comparative 4 226 A 2.4 48.31.005 Comparative 5 256 A 2.5 21.7 1.006 Comparative 6 226 A 2.4 25.21.006 Comparative 7 256 A 2.5 68.4 1.003 Comparative 8 409 Am 6.3 8.11.007 Comparative 9 255 A 2.5 17.0 1.006 Comparative 10 284 A 3.0 13.71.007 Comparative 11 158 A 1.6 10.2 1.007 Comparative 12 401 Am 5.5 15.91.006 Comparative 13 422 Am 6.5 14.7 1.006 *⁾A represents “anatase”; Amrepresents “amorphous”.

As shown in Table 2, the barium titanate powder obtained by the methodof the invention has a specific surface area in the range of 4 to 20m²/g and a c/a ratio of 1.008 or more. Thus, the barium titanate powderobtained by the method of the invention is comprised of fine particlessuperior in tetragonality.

Further, the method of the invention needs no calcination of bariumtitanate powder obtained by hydrothermal treatment of barium titanateprecursor, and accordingly the method of the invention usually providesbarium titanate powder superior in dispersibility without disintegrationof the resulting barium titanate powder.

1. A method for producing barium titanate powder comprising the stepsof: adding an aqueous slurry of anatase hydrous titanium oxide having aBET specific surface area in the range of 200 m²/g to 400 m²/g and ahalf width of diffraction peak of (101) plane in the range of 2.3° to5.0° as measured by X-ray diffraction to an aqueous solution of bariumhydroxide while maintaining the aqueous solution of barium hydroxide ata temperature in the range from 80° C. to the boiling point thereofunder normal pressure to cause a reaction of the barium hydroxide withthe hydrous titanium oxide to provide an aqueous slurry of bariumtitanate precursor; and subjecting the barium titanate precursor thusobtained to hydrothermal treatment over a period of not less than 24hours to provide barium titanate particles.
 2. The method according toclaim 1, wherein in the step of subjecting the barium titanate precursorto hydrothermal treatment to provide barium titanate, the hydrothermaltreatment is performed at a temperature of 250° C. or less.
 3. Themethod according to claim 1 comprising the steps of: adding an aqueoussolution of titanium tetrachloride and an aqueous solution of an alkalito water maintained at a temperature of 45° C. to 65° C. at the sametime while maintaining the temperature of the resulting reaction mixturein the range of 45° C. to 65° C. and maintaining the pH thereof in therange of 1.5 to 3.5 to simultaneously neutralize the titaniumtetrachloride with the alkali thereby to provide an aqueous slurry ofanatase hydrous titanium oxide having a BET specific surface area in therange of 200 m²/g to 400 m²/g and a half width of diffraction peak of(101) plane in the range of 2.3° to 5.0° as measured by X-raydiffraction; adding the aqueous slurry of anatase hydrous titanium oxidethus obtained to an aqueous solution of barium hydroxide whilemaintaining the aqueous solution of barium hydroxide solution at atemperature in the range from 80° C. to the boiling point thereof undernormal pressure to cause a reaction of the barium hydroxide with thehydrous titanium oxide to provide an aqueous slurry of barium titanateprecursor; and subjecting the barium titanate precursor thus obtained tohydrothermal treatment over a period of not less than 24 hours toprovide barium titanate particles.
 4. The method according to claim 3,wherein in the step of subjecting the barium titanate precursor tohydrothermal treatment to obtain barium titanate, the hydrothermaltreatment is performed at a temperature of 250° C. or less.